1. Field of the Invention
The present invention relates generally to ground penetrating radar and, more specifically, to a preferably digitally controlled transceiver operable to produce and broadcast a pseudo noise code through geological formations at medium frequencies.
2. Description of Prior Art
Ground penetrating radars generally require a carrier frequency of at least 50 MHz in order to provide a useable range resolution. This frequency is too high to penetrate many lithologies of interest. If the carrier frequency is greatly reduced to permit better penetration of the medium, then the bandwidth of the radar must also be reduced. Range resolution is directly related to the bandwidth, thus the resolution becomes poor and the radar may be useless. It would be desirable to provide a system wherein resolution was not dependent on bandwidth so that lower frequency transmission could be used.
Ground imaging radar systems also require system bandwidths much higher than possible in the down-hole environment. Another problem of the prior art is that multiple sensors or moving sensors are generally used for image forming and this is generally not possible or not practical at a subsurface location within a small diameter hole.
Most magnetically based detectors are used for metal detection. Such detectors are not generally useable for detection of downhole formation features and properties such as a water incursion many meters from the instrument because of a lack of sensitivity, poor resolution, and a weak response to water or other interfaces.
One use of a radar that could be transmitted at relatively lower frequencies for detecting formation interfaces would be to detect and locate the movement of approaching water to an oil producing well such as the water incursion caused by water injection that is used to assist in the extraction of oil. The detection of injected water in one or more production zones of the well would permit these sections of the well to be shut down in a timely manner and thereby allow for continued production from other zones of the well. If the water incursion is permitted to flow into the well through only one zone, then production from all zones may typically need to be stopped.
Another use of such a radar concerns a basic problem that has existed in the well logging business for decades, i.e., the effects of invasion that occur prior to running casing. During drilling, the drilling fluid which may be water or oil based is typically weighted to be overbalanced so as to prevent a blowout. The pressure at the formations of interest due to the overbalanced fluid column within the wellbore forces the drilling fluid into the formation and displaces the true formation fluids to produce what is commonly known as invasion. The invaded zone may typically extend into the formation by an amount usually less than about two meters. Because electronic logging tools are attempting to detect and measure the true formation fluids which have been displaced by an undetermined amount of drilling fluid, inaccuracies have, for decades, plagued the logging industry. The standard method for overcoming such inaccuracies is to use two or more sensors of the same or similar type that read different depths into the formation, e.g., dual laterolog tools, dual induction tools, and dual spaced neutron tools and use the difference in readings to correct for invasion. However, due to the unknown amount of change in resistivities, permittivities, electron densities, and so forth caused by the displacement of true formation fluid, and due to partial invasion regions wherein invasion is not complete, inaccuracies caused by invasion may be difficult to resolve. Therefore, it would be highly desirable to provide a downhole logging tool for logging prior to installation of casing that directly measures the depth of invasion to thereby provide a more reliable means for correcting the errors produced in logging tools due to invasion.
U.S. Pat. No. 4,937,580, issued Jun. 26, 1990, to R. H. Wills, discloses a ground probing radar for detecting radar reflections from underground objects. The radar is of the pulse compression type. A transmitter generates a biphase digitally modulated carrier signal. The digital modulations comprise successive pairs of complementary codes. Reflections of the transmitted signal from underground objects are demodulated and cross-correlated with the code words to produce a reflectivity sequence signal. The use of complementary codes results in minimal time sidelobes and improved range and resolution.
U.S. Pat. No. 5,904,210, issued May 18, 1999, to Stump and Allen, discloses an apparatus and method for determining a location and an orientation of an underground boring tool by employment of a radar-like probe and detection technique. The boring tool is provided with a device which generates a specific signature signal in response to a probe signal transmitted from above the ground. Cooperation between the probe signal transmitter at ground level and the signature signal generating device provided at the underground boring tool results in accurate detection of the boring tool location and, if desired, orientation, despite the presence of a large background signal. Precision detection of the boring tool location and orientation enables the operator to accurately locate the boring tool during operation and, if provided with a directional capacity, avoid buried obstacles such as utilities and other hazards. The signature signal produced by the boring tool may be generated either passively or actively, and may be a microwave or an acoustic signal. Further, the signature signal may be produced in a manner which differs from that used to produce the probe signal in one or more ways, including timing, frequency content, information content, or polarization.
U.S. Pat. No. 5,900,833, issued May 4, 1999, to Sunlin and Heger, discloses a ground penetrating impulse radar system providing three-dimensional images of targets. A moving array of transmitting and receiving antennas provides narrow beamwidths and high gain by real and synthetic aperture beam processing. Narrow pulsewidth impulse signals are utilized to obtain high resolution. Round trip time is calculated for each three-dimensional pixel in a search volume, and is used to process three-dimensional imagery. Analog to digital conversion can be utilized, so all signal processing is accomplished digitally. The radar system is applicable in detecting small objects near the surface by using very narrow pulses, and also can be applied to detect large, deep objects by wider pulses.
U.S. Pat. No. 5,869,967, issued Feb. 9, 1999, to Reinhold Straus, discloses a device for the detection of objects lying in the earth which, irrespective of topography, soil structure and state of the terrain, permits high surface yields with great precision in identifying the position of the objects to be detected without endangering the operating personnel. On a mobile device is arranged at least one jib that swivels about a vertical axis, on whose free end are arranged adjacent to each other several measuring heads for sweeping over strip-shaped surface areas of the terrain to be investigated. With the measuring heads on the free end of the jib at least one ground marking device is arranged for distinguishing the find site determined by the measuring heads.
U.S. Pat. No. 5,819,859, issued Oct. 13, 1998 (disclaimedxe2x80x94not to extend beyond the expiration date of U.S. Pat. No. 5,720,354), to Stump and Allen, discloses an apparatus and method for locating an underground object or structure by employment of a radar-like probe and detection technique. The underground structure is provided with a device which generates a specific signature signal in response to a probe signal transmitted from above the ground. Cooperative action between the probe signal transmitter at ground level and the signature signal generating device provided on the underground object provides for accurate detection of the subsurface object, despite the presence of a large background noise signal.
U.S. Pat. No. 5,659,985, issued Aug. 26, 1997 (disclaimedxe2x80x94not to extend beyond the expiration date of U.S. Pat. No. 5,553,407), to Greg Stump, discloses an information acquisition and control system and process that acquires geological information from a subsurface and modifies the operation of an excavating machine using the acquired geological information. The system includes a data acquisition system to acquire geological information along an excavation route, a machine controller to control the operation of the excavating machine, and a main controller that produces estimated machine performance parameters for use by the machine controller using the acquired geological information and machine operation information. In another embodiment, geologic characteristics acquired for a first subsurface are associated with excavation performance information for the first subsurface to produce correlation data. Geologic characteristics acquired at a second subsurface are compared with the correlation data to produce estimated excavation performance information for the second subsurface.
U.S. Pat. No. 4,965,582, issued Oct. 23, 1990, to Hans O. Hellsten, discloses an invention relating to a method for radar mapping an area and radar equipment for wideband exploration at frequencies below 300 MHz. A large number of frequencies, for instance 1000, are distributed over a frequency band between for instance 12.5 and 200 MHz, and approximately corresponding to terms in a geometrical series but being different harmonics to a certain fundamental frequency. This is accomplished by a synthesis generator coupled to a phase control device and the generated frequencies are each amplified in a separate amplifier, the outputs of which are guided in groups to a number of antennas, tuned to different frequency bands and fewer than the number of frequencies. The reception is carried out in a similar way from the antennas with pre-amplifiers and a mixer each and an A/D-converter and a registration device. The equipment is meant to use the principle of so called synthetic aperture radar (SAR).
U.S. Pat. No. 4,866,446, issued Sep. 12, 1989, to Hans O. Hellsten, is the parent patent to the above listed U.S. Pat. No. 4,965,582 discussed hereinbefore.
U.S. Pat. No. 5,933,014, issued Aug. 3, 1999, to Hartrumpf et al., discloses a process for detecting totally or partially hidden faults, such as cracks, and bubbles and the like in an opaque medium, by using microwave radiation. Microwaves from a transmitting antenna are directed against the surface of the medium which is to be inspected, and microwave radiation reflected or back scattered from the medium is detected and analyzed. In order to maximize the signal to noise ratio, minimizing the detection of radiation reflected by the surfaces of the medium itself, at least one of the transmitting antenna and the receiving antenna is oriented at an oblique angle relative to the surface of the medium.
International Publication No. WO 98/31209, published Jul. 23, 1998, to Ericksson et. al., discloses a forest vehicle provided with automatic planting assemblies mounted at hydraulically controlled arms. At the frontmost portion of the vehicle on a movable arm scanning means such as a georadar screen are provided, which transmit radar waves having a suitable wavelength and scan the ground area in front of the vehicle. The georadar determines the depth of soil and obstacle to planting such as stones and stumps.
The above prior art does not disclose a system that may be used in a borehole wherein operation may occur at medium frequencies and wherein resolution of formation features is not dependent upon system bandwidth. Therefore, those skilled in the art have long sought and will appreciate the present invention that addresses these and other problems.
An object of the present invention is to provide an improved system for detecting subterranean interfaces such as fluid interfaces or geological interfaces.
Another object of the present invention is to detect slow movement or changes in subterranean interfaces.
Yet another object of the present invention is to detect and locate the movement of approaching water to an oil producing well such as the water incursion caused by a water injection well that is used to assist in the extraction of oil.
A feature of one embodiment of the invention is a radar implemented as a downhole logging tool to contribute to the understanding of the lithography surrounding a borehole.
Another feature of a presently preferred embodiment of the invention is that a sampling speed of an analog to digital converter determines range measurement accuracy and range measurement resolution rather than the bandwidth of the system.
Another feature of a presently preferred embodiment of the present invention is a dynamic transmit/receiving gating that permits reflected signals that vary in magnitude over a very large dynamic range to be accommodated whereby the dynamic transmit/receiving gating operates so that only reflections from a particular range of distances are processed at any one time.
Yet another feature of one preferred embodiment of the present invention is a digital design that can be packaged and ruggedized to withstand the temperatures, pressures and size constraints of a down-hole environment.
An advantage of the present invention is the ability to map geological formation and fluid interfaces and track the movement of underground liquids such as in underground water reservoirs.
Yet another advantage of the present invention is the ability to produce very strong correlation peaks associated with long pseudo noise codes that can be used to discriminate against reflected signals from other ranges not currently being measured.
Yet another advantage of the present invention is great versatility and configurability to permit the same equipment to be used for a wide variety of applications.
These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. It will be understood that above listed objects, features, and advantages of the invention are intended only as an aid in understanding aspects of the invention, are not intended to limit the invention in any way, and do not form a comprehensive list of such objects, features, and advantages.
Therefore, the present invention comprises a method for detecting interfaces in an earth formation that comprises such steps as generating a plurality of digital pulses, producing the plurality of digital pulses at a carrier frequency, and transmitting the plurality of digital pulses into the geological formation from a first physical position. A received signal is comprised of reflections of the plurality of digital pulses from the geological formation. The received signal is oversampled with a sampling rate that is greater than five times the carrier frequency to produce a digitized received signal. A gate may be used to select a portion of the digitized received signal during a selected time frame of the received signal to thereby provide a gated digitized received signal. The plurality of transmitted digital pulses are correlated with the gated digitized received signal for detecting an interface at a selected distance from the first physical position.
The present invention permits utilizing a carrier frequency in the range from 100 KHz to 1 MHz. The plurality of digital pulses may be accumulated over a selected period of time prior to the step of correlating. In a preferred embodiment, a digital to analog converter is provided for converting the plurality of digital pulses at the carrier frequency to an analog signal prior to the step of transmitting. In this case, the step of producing of the plurality of digital pulses at the carrier frequency may be completed entirely with digital components. The step of oversampling may further comprise oversampling the received signal such that a sampling rate is used that is from five to fifty times the carrier signal.
In a preferred embodiment of the invention, the first position may be within a borehole through the earth formation or adjacent thereto. The method may then further comprise transmitting the plurality of digital pulses from a dipole antenna within the borehole. Moreover, the method may comprise providing a plurality of dipole antennas within a downhole tool. The plurality of digital pulses may be transmitted through a non-ferrous wellbore tubular.
In other words, the invention may provide a downhole system for detecting interfaces in an earth formation, the downhole system may be operable in a borehole within the earth formation. In this case, the downhole system preferably comprises an all digital signal generator for producing a plurality of digital pulses at a carrier frequency and at least one dipole antenna associated with the downhole system for transmitting a transmitted signal and receiving a reflected signal. The analog to digital signal converter is then provided for sampling the received signal at a sampling rate more than five times the carrier frequency to produce a received digital signal. The correlator compares the plurality of transmitted digital pulses with respect to the received digital signal.
To facilitate an all-digital transmitter system, a digital to analog converter may be provided for converting the plurality of digital pulses at the carrier frequency to an analog signal. The carrier frequency may be selectable and may, unlike other georadars, be selected to be less than 1 MHz. In fact, the carrier frequency may be between 100 KHz and 1 MHz. The gate control for selecting a portion of the received signal related to a selected distance from the borehole effectively acts as a control for selecting the distance from the borehole at which a search for an interface takes place.
A simulator is also provided for simulating a radar signal transmitted into a geological formation. The simulator comprises a pseudo code generator for producing a transmitted signal comprising a plurality of digital pulses and a plurality of selectable time delay elements. A filter is provided having a response related to the geological formation and a gating control is used for selecting a gated portion of a returned signal related to a selected distance into the geological formation.
In a preferred embodiment, the filter further comprises a Butterworth bandpass filter. A plurality of selectable attenuators are provided for respective of the plurality of selectable time delay elements. A noise generator is used to produce noise in the simulated received signal. An analog to digital converter is used in the simulator for receiving the transmitted signal from the filter to produce the returned signal. An accumulator stores the gated portion of the returned signal.
A method is also provided for controlling a producing zone in a geological formation of a well having a plurality of producing zones. The method comprises steps such as transmitting a plurality of pulses into the geological formation and receiving the reflections from the plurality of pulses due to a waterfront approaching within the producing zone. The waterfront is detected from the reflections and the producing zone is closed down. The method may typically involve selectively controlling the carrier frequency at frequencies below 1 MHz. The method typically requires selecting the one or more reflections from the waterfront by providing a delay. The detected reflections are then correlated with a stored signal containing a sequence for the plurality of pulses.
An antenna is positioned within the well for the transmitting of the plurality of pulses into the geological formation. A non-ferrous wellbore tubular such as plastic casing or liner permits the transmitting of the plurality of pulses into the geological formation from within the well.